I imagine you would know it was a Green Drake if you saw one, assuming you saw one like I saw. I don't know what size it would equate to, but it probably had a 3" wing span. I've seen a lot of different hatches, and this bug absolutely dwarfed anything I had seen before. It seemed like it was at least twice as big as any other bug I have seen hatch on the Davidson. I'm sure I'm not remembering it as accurate as it actually was, but I'm pretty sure I don't have any dries in my box that would equate to the size of the bug. It was big.

I was the "customer" that let Byron know about the Drake hatch. We just got lucky and were there at the right time to see it.
They were definitely green drakes
I did see fish taken them off the surface but we caught all of our fish on March Browns

I love it when you are at the right place at the right time with this hatch. Fish don't seem to pay much attention the dun. I think it's because the fly sheds it's exoskeleton underwater and emerges fully formed. Fish do go crazy over the returning females when they return to lay their eggs. I use an imitation called a Dette Coffin Fly with good success then. It's fun to be in the middle of the stream about dust and hear what sounds like someone throwing rocks the size of your head in Here is a photo of my version of the Dette Coffin Fly.